DE2608089C3 - Method for producing a superconducting Nb3 Sn layer on a niobium surface for high frequency applications - Google Patents

Description

The invention relates to a method for producing a superconducting NbiSn layer on a niobium surface for high-frequency applications by indil'fiindicren of tin in this surface at increased Tempera door.

Superconductive devices for operation with high frequency electromagnetic oil ^; Elder with frequencies of about 10 MHz and more can be used in technology in a variety of ways. In particular, they can be used as resonators and separators for particle accelerators or as high-frequency resonators for other purposes, for example as sequence standards, and in particular be designed as cavity resonators or as Rcsonalor coils. Superconducting cavity resonators are operated in the frequency range from 1 to 15 GIIz, superconducting resonator waves in the range of MHz. Up to now, mostly niobium and occasionally also lead have been used as superconducting materials for such resonators.

In such superconducting devices, the aim is to achieve a high quality and, as a rule, the highest possible critical magnetic flux density B ', "" measured under the action of hole frequency fields, in order to be able to operate the superconducting devices with the highest possible frequency performance with at the same time low surface resistance If the critical magnetic flux density ß '"is exceeded, the losses increase sharply. the surface resistance increases considerably and the electromagnetic field collapses. An upper limit door B '" is given by the so-called thermodynamically critical flux density S _1. Since the thermodynamically critical flux density B ₁ - of Nb ₁ Sn is higher than that of niobium, it is to be expected that a higher critical flux density B '" can be achieved than on a niobium surface. Furthermore, NbjSn also has a significantly higher critical temperature than niobium, so that on the one hand it has a higher thermal stability and on the other hand it should also allow higher operating temperatures than niobium, in particular operation at the temperature of the boiling liquid helium of 4.2 K, which is necessary for High frequency applications of niobium is already too high.

Attempts have also already been made to apply thin protective layers of NbiSn to niobium resonators by first evaporating tin onto the niobium resonator and then heat-treating it. With such surface layers, a quality Q ₁₁ of about 10 ¹¹ and a critical flux density ß ', "of about 25 niT were measured at 2.8 GHz (cf." Siemens-Forsehungsiind Fmwickliingsberichic "J [1974], page 96, calculated column).

Furthermore, it is already known to place the niobium parts to be provided with an NbiSn-Sehichi in a closed reaction vessel, namely a closed, evacuated Q / iiarzampulle. exposing a Zinndampfatmosphärc at an elevated temperature of about 1000 "C, from which the tin to form the desired NbiSn layer eindilTundiert in the niobium surface. Here, Nb blank ₁ Sn ■ layers of several microns thickness and already relatively good properties, for example with grades Qu of about H) "and critical magnetic!"'ulklichien BT I of eiwas over 4OmT at 1.5 K. achieve (paper by Hillenbrand, inter alia, in "IEEE Transactions on Magnetics", Vol. MAG-I no. 2 March /. 1975 Pages 420 to 422).

The invention is based on the object of producing superconducting NbiSn joints To further improve niobium surfaces for block frequency applications. In particular, there will be another The aim is to increase the quality and the critical magnetic density of the NbiSn layers produced, where it is particularly important to have high grades at temperatures that are necessary for high frequency applications of niobium are already too high, especially at the temperature of the boiling liquid helium of 4.2 K.

To achieve this object, according to the invention, in a method of the type mentioned at the outset, such a method is used proceeded that the niobium surface to be provided with the NbjSn layer first oxidized anodically is, then the niobium oxide layer produced in this way is introduced into an evacuable reaction chamber and there after evacuation is exposed to a tin vapor atmosphere in such a way that on the niobium oxide layer forms a tin layer, and finally the niobium surface to form the NbiSn layer at a temperature

Γ)

between 930 and about 1400 "C / t is raised.

The niobium oxide layer initially produced on the niobium surface by anodic oxidation in the method according to the invention - it is an amorphous layer made of niobium pentoxide - has an extremely advantageous effect on the subsequent formation of the NbiSn layer. In particular, can form a uniform layer of tin on the niobium oxide layer, which diffuse in the subsequent heating to the temperature between 930 and about 1400 ⁰ C in the niobium surface and by reaction with the niobium STHR a uniform NbjSn layer can be formed. In the event of a comparison attempt. in which a non-anodized niobium surface was exposed to a tin vapor atmosphere, after the final heat treatment to form the NbiSn layer, however, it was found that many crystal grains of the niobium surface were poorly coated with NbiSn and that some niobium surfaces were still bright. Apparently, the niobium oxide layer, which dissolves at a temperature between 500 and 600 ° C, largely compensates for the different influences on the nucleation during the formation of the NbiSn layer, which occur on a bare niobium surface due to its surface grain structure.

It is particularly cheap. when heating to form the NbiSn layer in the presence of a Source of tin takes place. This can lead to excessive evaporation of tin from the niobium surface avoided and possibly additional tin to form the NbiSn layer from the tin vapor atmosphere to be delivered later.

The method according to the invention can be carried out in this way in a particularly simple and therefore advantageous manner. that the anodized niobium surface is evacuated together with a tin source Reaction chamber introduced and this is heated after evacuation in such a way that initially to Formation of the tin layer on the niobium oxide layer puts the tin source at a higher temperature than the niobium surface and then the niobium surface and the tin source to about the same temperature between 930 and about 1400 "C. The formation of the tin layers on the niobium oxide layer and the subsequent production of the NbiSn layer can be used this embodiment of the method according to the invention in immediately successive steps take place in the same reaction chamber.

The temperature range between 930 and 14 (M) "C isi particularly favorable for the formation of the NbiSn layer. Below 930 "C there is a risk that undesired / internal phases of the niobium-tin system are formed. Above 14 () 0 "C can on the other hand, it is difficult to control the layer growth.

It has proven to be particularly favorable with regard to the quality and the critical magnetic flux density of the NbiSn layer formed to initially move the anodized niobium surface from room temperature to a temperature between about 30 minutes to 2 hours, preferably about 60 minutes 500 and 600 "C and the tin source from room temperature to a temperature between 800 and 900" C, then further heating the niobium surface and tin source to a temperature between 1000 and 1100 ⁰ C in a period of about 20 to 40 minutes and then to form the Nb) Sn-Schiehi at this temperature

lulled. | c hereby selects the first Fj-hizungsphusL ' is chosen, the longer should the / wide heating pluisc last. The time it takes for the niobium surface and tin to form the NbjSn layer on the after the second phase reached temperature / between 1000 and 1100 C is kept at thick NbjSn layers last up to 100 hours.

You can use: ;; according to the method ili «. · Coating of the niobium oxide layer with a inner layer and the subsequent heat treatment to form the NbjSn layer in an evacuated, closed, for example melted R ^ action chamber carry out. However, it is even cheaper to have one opposite in an evacuable reaction chamber delimited from the rest of the chamber volume, the tin source and that to be provided with the NbjSn layer Niobium surface containing reaction area to bil to seal the and these against the rest of the chamber volume so far that when evacuating the Reaction chamber existing or occurring gases within the reaction area from this be sucked off, but during later heating the tin vapor pressure within the reaction area remains increased compared to the tin vapor pressure in the remainder of the chamber, and during heating and the subsequent heat treatment to continuously pump out the reaction chamber.

This procedure, in which an open reaction chamber is used and which has already been proposed in a similar form in German patent application P 25 32 570.6-45. has numerous advantages over working in a closed reaction ampoule. First of all, an open reaction chamber can be used over and over again, while a closed reaction ampoule usually has to be destroyed when it is opened. Furthermore, when working in an open reaction chamber, the gases released from the surface of the heated parts or exiting the heated parts during the course of heating are constantly pumped out, while when working with the ampoule closed, the gases produced during or after melting remain trapped in the ampoule and can lead to disturbances of the NbjSn layer. This is achieved through the formation of the reaction area which is sealed off from the remaining volume of the reaction chamber. despite the open reaction chamber during the formation of the tin layer on the niobium oxide layer and the subsequent formation of the NbjSn layer, avoiding too much tin from diffusing away in the direction of the cold end of the reaction chamber and the pump connected there. It is particularly cheap. when the niobium surface to be provided with the NbjSn layer itself forms a boundary surface of the reaction area. Quartz, for example, can be used to further limit the reaction area. In order to avoid a reaction of tin with the quartz walls and the incorporation of silicon into the NbjSn-layer, but when using quartz, a reaction temperature of 1050 ⁰ C should not be exceeded as possible. However, such difficulties can advantageously be avoided by using only niobium surfaces or anodically oxidized niobium surfaces to delimit the reaction area. One or more of these niobium surfaces can in turn be the surfaces to be coated with the NbjSn layer themselves. The sealing of the reaction area is advantageously carried out in this method in such a way that the Gcsamiqucrschnii the connecting paths between the interior of the! sealed reaction area and the most eastern RcaL'.ior.skamniei. ie the total cross-section of the reaction area is smaller than the surface of the tin source, i.e. that surface of the molten tin supply from which the tin evaporates in the reaction area. With this dimensioning de; Leak cross-sections can then only ah less

κι the echo of the evaporating tin from the return area Pass into the reaction chamber. Particularly favorable in terms of a further reduction the tin losses, however, are when the total leakage cross-section does not exceed 25% of the surface.

r> before the tin source amount ». In most cases, however, the total leakage cross-section cannot be precisely determined; rather, one has to rely on estimates. With a total leak cross-section in the order of 0.1 cm ² , a very good one is still achieved

2 (i evacuation of those within the reaction areas! existing or occurring interfering gases, d; these have a much lower molecular weight a1; Tin and are therefore pumped out much more quickly than, for example, tin vapor.

2) Save from the surface of the tin source and from The leakage cross-section of the reaction area is dci tin vapor pressure but also from the size of the mi The niobium surface to be provided on the NbjSn layer is determined, since the deposited on the niobium oxide layer

in or after dissolution of this oxide layer with increasedi Temperature in the niobium diffusing tin is removed from the vapor space. To not au; for this reason too low a tin vapor pressure in the reaction space and thus too long reaction times zi

r> obtained, the surface of the tin qucllc should be possible greater than 0.2%. preferably greater than 1% of the niobium surface to be provided with the NbiSn layer to get voted. Any other niobium surfaces delimiting the reaction area are, as far as don

4 (i can form NbiSn layers, corresponding to zi consider.

During the heating up and the subsequent reaction time, the reaction chamber will of course be evacuated as well as possible. It has proven to be advantageous to pump out the reaction chamber and there; Carry out heating of the reaction area in such a way that during heating and the reaction time eir residual gas pressure of 10 ^A Torr, measured at the cold end of the reaction chamber connected to the pump

-, ο is not exceeded or is only exceeded for a short time.

The niobium pentoxide layer to be produced by anodic oxidation on the niobium surface to be provided with the NbjSn layer should preferably be 0.01 to 0.3 μm thick. Oxide layers with a thickness of less than 0.01 μm actually show practically no advantageous effect on the properties of the Nb ₃ Sn-SChJcIIt to be formed later, while when a layer thickness of 0.3 μm is exceeded in the case of anodic oxidation, slightly undesirable oxide

bo of gray color may form. Particularly cheap for there; The method according to the invention is a 0.04 to 0.15 μίτ thick niobium pentoxide layer for anodic oxidation the method known from DT-PS 21 06 62i, in which the anodic oxidation ir

_h5 an aqueous ammonia solution, preferably with 2 (up to 30 wt .-% ammonia) is carried out.

For the finished NbjSn layer, a layer thickness of about 0.5 to 5 μτη is favorable, which by corresponding

Dimensioning of the first line / single line / in formation of the NH ₁ Sn layer can be set. For one thing, such scissors are thick enough. that the electromagnetic currents and currents penetrate only into the NbiSn layer and not into the niobium layer below. Otherwise, the quality and the critical magnetic density of the surface in particular would not be determined by the NbiSn layer but by the niobium layer underneath On the other hand, the NiSn layers of the mentioned thickness are so thin that the residual heat generated in the NiSn layer takes a very short path to the niobium, whose thermal conductivity is higher than that of the NiSn, and from there to that during operation of the Execution of coolant in contact with the niobium body can be derived.

The high-frequency properties of the NbjSn layer formed can be improved even further if an oxide layer is produced on the NbiSn layer produced by anodic oxidation and is then removed again chemically. The creation and detachment of the oxide layer can also be repeated several times! will. An aqueous ammonia solution with 20 to 30% by weight of ammonia is suitable for instantaneous generation of the oxide layer, and around 40 to 50% hydrofluoric acid for chemical detachment of the oxide layer (cf. W ⁷ AlV. Transactions on Magnetics. Vol. MAG- 11. No. 2. 1975, pages 420 to 422 and DIi-OS 24 28 867).

The method according to the invention will be described in more detail with the aid of a few figures and exemplary embodiments explained.

Fig. 1 schematically shows an excerpt from a method for carrying out the method according to the invention suitable rcaklionskamnicr;

FIGS. 2 and 3 schematically show an exemplary one Embodiment of a device for performing the inventive method in two different ways Procedural stages;

Fig. 4 shows the course of the temperature of Niobium surface and tin source depending on the Time in an advantageous embodiment of the method according to the invention.

FIG. 1 shows a pot-shaped niobium part 1 for a conventional circular-cylindrical cavity resonator of the TE ₁ III field type for a frequency of 9.5 GHz in the A-band range, the inner diameter and height of which are each 41 mm. The production of a superconducting NbjSn layer on the inner surface of this pot-shaped niobium part 1 will be explained in detail below as an exemplary embodiment of the method according to the invention.

The starting material used for the niobium part 1 was niobium of the usual purity (reactor quality, purity> 99.8%), which was cold-worked by the manufacturer and recrystallized to small grains. The Niobteil 1 has been rotated from the solid and then annealed in ultra high vacuum at a temperature of 1900 ⁰ C for 50 hours. The mean grain size was then about 5 mm. Then an approximately 150 μm thick layer was removed from the surface of the niobium part on all sides by anodic polishing with an oscillating current. The polishing process, in which preferably with an electrolyte of 89.0 to 90.5 wt. O / o H ₂ SO ₄ , 2.2 to 3.0 wt. O / o HF and the remaining parts by weight of H2O at one temperature from 20 to 35 ° C and with constant voltages between 11 and 13 V is used in the DE-PS

20 27 15b described in detail. After the anodic Another 70 μm thick light was polished by chemical polishing in a polishing solution from one part concentrated nitric acid and one Part 4 () "/ (liquid liquid acid removed. Both polishing steps were used to remove those niobium surface layers. due to the previous drchen Exhibited disturbances in their structure. After polishing, the cup-shaped niobium part 1 was very smooth Surface on.

The inner surface of the niobium part 1 was then coated with a niobium oxide layer 2 by anodic oxidation. The niobium part 1 itself was used as a vessel for the oxidation bath and was filled to the brim with an aqueous ammonia solution containing 25% by weight of ammonia. The niobium part 1 was then connected to the positive pole of a voltage source. A niobium tube with an outside diameter of about 20 mm and about 30 mm deep, coaxially to the cylinder axis of the niobium part 1, was immersed in the bath as a cathode. The bath temperature was about 20 ° C. The anodic oxidation took place with a constant voltage of 50 V between anode and cathode. The initial current density on the inner surface of the niobium part I was approximately ΙδηιΑ / αιι ² . The current decreases with increasing oxide layer thickness. The oxidation process was terminated at a current density of about 1 inA / cm ^2. At this low current density, the voltage drop in the electrolyte is small, so that the voltage of 50 V practically drops across the oxide layer 2, which then has a uniform thickness of about 0.1 μm. After emptying, the niobium part was then rinsed with distilled water.

To produce the NbiSn layer, the niobium part 1 was placed in a quartz tube 3, which forms a reaction chamber that can be evacuated. The niobium part 1 was placed on a lower part 4 also made of niobium, in the center of which there was a depression 5 into which a tin supply 6 (pa quality, purity> 99.96%) was introduced. The surface of the tin supply 6 introduced into the depression 5 can advantageously be approximately 2 cm ³ in the molten state, ie approximately 3% of the niobium surface to be provided with the NbjSn layer. The niobium part 1 and the niobium lower part 4 form a reaction area which is delimited from the remaining volume of the quartz tube 3 and which contains both the tin source 6 and the anodically oxidized inner surface of the niobium part 1 to be provided _{with the Nb 3 Sn layer.} The end face of the niobium part 1 simply rests on the surface of the niobium lower part 4. The unevenness of both surfaces have a maximum depth of about 50 μm. As a result, a gap is maintained between the two surfaces which is sufficient to sufficiently evacuate the interior of the reaction area enclosed by parts 1 and 4, but is again so small that during subsequent heating the tin vapor pressure within the reaction area compared to the tin vapor pressure in the remaining parts of the quartz tube 3 remains increased. The total leakage cross-section between the interior of the reaction area and the remaining part of the quartz ampoule 3 is approximately 0.15 cm ² .

In order to be able to provide a second resonator with an Nb.iSn layer in the same operation, a second niobium base 7 with a tin source 8 was placed on the niobium part 1, and another one on top of it pot-shaped niobium part 9 was placed. The niobium parts 7

and 9 games for the wide: Explanation of the There will be no resolution according to the procedure however mentioned. d; i them because of their heat-mediating properties Effect the in I i g. 4 have influenced the temperature trend in niobium parts I and 4.

Those from the niobium lines! 4, 7 and 9 existing turmförmigc assembly was mounted in the Quar / vial i on a Quarzrohrsliiek 10th Before installation, all niobium parts were rinsed in acetone.

Like the l · ι g. 2 and 3 / own, was also included in the Quar / ampullc 3 yet another, mil Quar / wool filled, evacuated and melted sealed quar / ampullc Il inserted and milicls a quartz tube piece 12 placed on the niobium part 9. In order not to hinder the evacuation of the lower part of the quartz ampoule 3, the quartz ampoule contained 11 one in FIGS. 2 and 3 shown in dashed lines, extending in the axial direction opening and closed also not tight with the inner wall of the quartz ampoule 3 from.

The quartz ampoule 3, as shown in FIG. 3 shows a stainless steel flange 14 with a Edelstahlsehlaueh 15 connected via an indium seal 13 in order \, leading to a turbo molecular pump. The quartz ampoule 3 was then evacuated at room temperature until a pressure of 5 · 10 ^s Torr prevailed at the end of the hose 15 on the pump side. In the meantime, a vertical, tubular resistance heating furnace 16 was heated to a temperature of 10 + 5 ° C. The furnace was closed at the bottom with quartz wool.

To Deginn of the Beschichiungsvorganges was Quar / ampullc 3 lowered so far into the furnace 16 with the turbo molecular pump running that the The upper edge of the niobium lower part 4 ended with the upper edge of the furnace 16. In this in F i g. 2 shown The arrangement was left in place for an hour. During this time, the niobium parts heated up 4, 1, 7 and 9 on different temperatures decreasing from bottom to top. Because the conduction is better within the niobium parts than at the transition points between the niobium parts, it turned out essentially a different one in each of the four parts, but somewhat homogeneous within the respective part Temperature.

4 shows the temperatures measured on the niobium parts 1 and 4 during the heating process. ^{The temperature 7Ίη 1} C is plotted on the ordinate and the heating time / in minutes is plotted on the abscissa. The temperature of the niobium lower part 4 and thus also of the tin source 6 is shown by the curve ;;, the temperature of the lobed niobium part I by the curve b . As is apparent from Fig. 4, it turned after about 12 minutes between the both-τ parts of a maximum temperature difference of about 600 "C, which decreased during long lasting heating again. After 60 minutes the temperature was of Niobunterleils 4 about 830 ⁰ C and the temperature of about 570 Niobteils 1 ⁰ C. During this time, from the Zinndampfatmosphäre, the check on heating in the space enclosed by the parts 1 and 4 forms space is deposited on the niobium pentoxide 2 a tin layer. It is important for this deposition process, that the tin source 6 is at a higher temperature, namely the temperature of the niobium lower part 4, than the inner surface of the niobium part 1.

After the mentioned heating time of 60 minutes, the quartz ampoule 3, as shown in FIG. 3, was so deep in the oils 16 lowered, that see the niobium parts I, -4, 7 and

9 in the homogeneously heated miniature part of the oven 16 found. Quar / anipulle filled with the mil quar / wool 11 it was ensured that practically neither

• ι the inner walls of the furnace 16 are still preserved Niobium parts could radiate heat into the space outside the furnace. It was achieved in such a way that see the After some parts of the niobium, an almost homogeneous temperature distribution was achieved. whereby in particular

Hi ie the Zinuquelle 6 in Niobunlteil 4 practically on the the same temperature as the niobium part I. As I i g. 4 shows this homogeneous temperature vericilting turned out about 25 minutes after lowering the Quartz ampoule 3 in oven 16. than in the niobium parts

ι "> I and 4 a uniform temperature of about 99" / O des Final value of 1050 C was reached. The niobium parts were then in the middle egg for another 3 hours! of left at 1050 "C heated oven. During this /.eil was on the inner surface of the niobium part I dl ·. ·

_> o desired NbiSn-Sehichl formed. To deginn this Time increased the pressure on the already mentioned Mclislellc near the turbo molecular pump to about IO ' Torr on. At the end of the 3 hour period it was still about 5 · 10 "goal /. It is important that you stay away during this

_ '"i response time the tin source 6 does not have a higher Temperature than the inner surface of the niobium I is located. Otherwise it is possible to use the steam The inner surface of the niobium part I condenses and drops of tin form there. When comparing tests was

ίο this undesirable! ": · observed as soon as the effect Temperature of the / innquellc 6 during the reaction time of the temperature of the Niobieeil I by more than

10 C deviated upwards.

At the end of the three hour reaction time

The furnace 16 was switched off, the Quar / wool 17 from lower end of the oven removed and a gentle stream of air from the bottom up through ilen oven blown. Here the oven cools down from below above, and it was achieved that the niobium part I.

κι was a little warmer than that while cooling down Niobium base 4 and the tin source 6. This in particular causes condensation of tin dip on the Inner surface of the niobium I avoided during cooling.

Γι After a cooling / eil of 1.5 hours, the Quartz ampoule Ϊ pulled completely out of the oven. The pressure at the measuring point near the turbo molecular pump at this point was still about 3 · 10 'Torr. The Niobicilc 9, 7, I and 4 showed in the

"in the order mentioned at this time clear red heat to barely visible red heat. After a further 1.5 hours the pressure was close to Turbomolecular pump only 2.5 ■ 10 «Torr.

After cooling down completely, the quartz chamber was

M pulle 3 vented with argon and opened. That on his Inner surface now an approximately 1 [im thick NbiSn layer having niobium 1 was then cleaned only with acetone and then together with a usual coupling part made of niobium with from below in the

Wi resonator cavity opening coupling lines in a Kryostatcn installed, as is known from DE-PS 21 64 529. Then the cavity resonator evacuated to about 3 · 10 "Torr using a turbo molecular pump and then initially with

h · -, liquid nitrogen and then with liquid Helium cooled. At 4.2 K the idle quality was Q> measured at low magnetic induction S = O. By lowering the pressure above the helium bath

Il

a temperature of 1.5 K cingosielh and, in both cases, 1 emporium "the flow rate Qn at low induction / i = 0 and at the highest possible induction H '" was measured.

The values obtained here for the critical magnetic induction Ii ", ' at which the field / usammciibnich stiiitlindci, and for the idle energy Qt .. the latter converted to a single level of the coupling part on its entire inner surface with Nb, Sn hc- Chiclitelen resonator, are given in the following table within experimental unit I.

Tabel

Experimental 4.2 K Ι.ό Κ 10 " I /) i < V Ii, " [ No. /; m> H = O 10 " H = / 10 " 1.4 ■ 10 " 2.1 · 10 " Kb 10 " 1.1 K 1.6 ■ 10 " 6.0 · 10 " 2.i ■ 10 " I. i, 5 ■ ιU " 2.0 · 1.4 ■ 10 ' 7 3.5 2 1.1 - 10 " 1.9 Kl 101.0 5 89.5 4th 101.0

After completing these measurements and removing the resonance from the cryosate, the with the Niobieils I. gradually remove thin layers by adding / next by anodic oxidation in an aqueous ammonia solution with about 25 wt .-% ammonia a Oxide layer is generated and then in 40 "/ oiger I was replaced by hydrofluoric acid. On the one hand, this procedure is repeated on a regular basis is repeated, in the DIi-OS 24 28 8f> 7 described.

After an approximately 0.18 .mu.m thick Nb ₁ Sn layer had been removed, the measurements described above were repeated and the results given in the table under Versuelis no. 2 values given.

After these tests were completed, the NbiSn layer from the inner surface of the niobium part I by chemical polishing in a one-part solution concentrated nitric acid and one part 4 ()% iger oleic acid completely removed. After a Policr / eii The NbiSn-Schichl was about 0.3 minutes away. Since the niobium surface is then not very was smooth. chemical polishing was ionized to a total length of 20 µm.

Subsequently, the niobium part 1 freed from the NbiSn layer was again anodically uidieri on its inside and again provided with an NbtSn-Si layer in the same way as is shown on the basis of FIG. I until 4 has already been explained. The niobium I coated in this way was again only rinsed with acetone after removing it from de ι Qiiarzampuiic 3. as already explained. embedded together with a niobium head in a crown. IV. · Ι eic, the subsequent measurement were the values given in the "dragonfly under test no. 3.) Values.

After this measurement, the NbiSn surface was again removed by repeated repeated oxidation and subsequent chemical detachment; a 0.12 µm thick layer was removed from the layer. Measurements were then taken again. animals Lrgebnk iv. > ier table under test no. 4 is listed. These last-mentioned machines were admittedly somewhat affected by dust, as a result of construction work in the laboratory building when a deep cavity resonator was set up in its interior.

As the aufgefühnen in animal experimental results table / inherent in weitlen cavity) '!!, after the erlintlungsgemäßen process with Nb, Sn layers were vcisehen, very high at 1.5 K while for the Q' and W ₁ ' achieved, which could be increased even further by anodic oxidation and subsequent chemical detachment of the oxide layer. However, it is of particular importance that) tlas according to the invention also delivers quality Q ₁ for temperatures of 4.2 K, above K) ". This means that, according to the method according to the invention, cavity resonators provided with NbiSn layers can also be used in temporaries where a niobium salt is no longer possible for high-frequency applications. Measurements of the quality Q _it at higher magnetic flux densities and measurements of the critical magnetic flux density B '. "Were not possible in the experiments explained above at 4.2 K because of the niobium coupling part. Schichl Resonatortopf provided also provided with a Nb, SN-Schichl Koppelleil was used, a critical magnetic Flußdichtc Ii ", 'of 78 nit was obtained at 4.2 K.

In addition to resonators of the Ti _1n type, other resonators, for example those of the TMniii type or resonator coils, can of course be provided with NbiSn layers with the aid of the method according to the invention.

llier / u 2 Ulan Zoiclimnmen

Claims (15)

Patent claims: 1. Method for producing a superconductive NbjSn layer on a niobium surface for High-frequency applications due to the diffusion of tin into this surface at increased levels Temperature, characterized that the niobium surface to be provided with the NbjSn layer is first anodically oxidized, then the niobium oxide layer produced in this way is introduced into a reaction chamber that can be evacuated and there is exposed to a tin vapor atmosphere after evacuation in such a way that on the Niobium oxide layer forms a layer of tin, and finally the niobium surface to form the Nb) Sn layer is heated to a temperature between 930 and about 1400 ° C. 2. The method according to claim 1, characterized in that the heating to form the NbiSn layer is made in the presence of a source of tin. 3. The method according to claim 2, characterized in that that the anodically oxidizable niobium surface together with the tin source in the evacuable Reaction chamber introduced and this is heated after evacuation in such a way that initially to Formation of the tin layer on the anodically oxidized niobium surface, the tin source at a higher temperature than the niobium surface and then the niobium surface and the tin source to about the same temperature between 930 and about 1400 ° C can be brought. 4. The method according to claim 3, characterized in that initially within a period of about 30 minutes to 2 hours, the anodically oxidized niobium surface from room temperature to a temperature between 500 and 600 "C and the tin source from room temperature to a temperature between 800 and 900" C to be heated and. are heated and then niobium surface Zinnquclle over a period of about 20 to 40 minutes at a temperature from 1000 to 1100 ⁰ C and further kept for forming the NbiSn layer at this temperature. 5. The method according to any one of claims 2 to 4, characterized in that in an evacuable reaction chamber, a reaction area delimited from the rest of the chamber volume and containing the tin source and the anodically oxidized niobium surface to be provided with the NbjSn layer is formed is sealed far that when evacuating the reaction chamber within the reaction area existing or occurring 'gases are sucked out of this, but during later heating the tin vapor pressure within the reaction area remains increased compared to the tin vapor pressure in the rest of the chamber, and that during the heating and the _t subsequent to the heat treatment, the reaction chamber is continually pumped out. 6. The method according to claim 5, characterized in that the with the NbiSn layer to provided niobium surface itself as a lie, tion area of the reaction area is provided. 7. The method according to claim 5 or 6 characterized that the reaction area only from Niobium surfaces or anodically oxidized niobium surfaces is limited. 8. The method according to any one of claims 5 to 7. ■ j characterized in that the overall cross section the communication paths between the interior of the sealed reaction area and the remaining reaction chamber is selected to be smaller than the surface of the tin source. ίο 9. The method according to claim 8, characterized in that that a total cross-section of the connecting paths of at most 25% of the surface of the Tin source is chosen. 10. The method according to any one of claims 5 to 9, l) characterized in that the surface of the Tin source greater than 0.2%, preferably greater than 1%, of that to be provided with the NbjSn layer Niobium surface is chosen. 11. The method according to any one of claims 5 to 10. characterized in that the pumping out of the Reaction chamber and the heating of the reaction area takes place in such a way that a residual gas pressure of 10 ⁴ Torr, measured at the cold end of the reaction chamber connected to the pump, is not exceeded or is only exceeded for a short time. 12. The method according to any one of claims I to 11, characterized in that by the anodic oxidation on the with the NbiSn layer providing niobium surface a 0.01 to 0.3 μm is generated in thick niobium pentoxide layer. 13. The method according to claim 12, characterized characterized as being 0.04 to 0.15 µm thick Niobium pentoxide layer is generated. 14. The method according to any one of claims I to 13. π characterized in that the niobium surface as long as the temperature / wipe 930 and about 1400 ° C until the niobium surface an Nb ^ n layer with a thickness between 0.5 and 5 μm is formed. (ι 15. The method according to any one of claims 1 to 14, characterized in that on the NbiSn layer produced by anodic oxidation Oxide layer is generated and then chemically removed again.